1. Field of the Invention
The present invention relates to prevention of reproduction of a document including a ground pattern added for preventing a reproduction thereof.
2. Description of the Related Art
Recently, electrophotographic copying machines have been spread widely, and characters or an image printed on a sheet of paper or the like can easily be copied by any person by using such a copying machine. Image processing technology and image forming technology are also improved significantly. Then, especially, by using a state of the art digital color copying machine (MFP), a copy difficult to be distinguished from an original can be created easily. Therefore, it is important to prevent forgery, falsification or illegal copying of prints of important documents.
In one of the measures for preventing forgery, falsification and false copy of a print, a ground pattern for preventing or deterring copying is added to an image of a document when it is printed by a printer having a function for outputting the ground pattern. An image of the document is read by the scanner when the document is copied, and when the ground pattern is detected in the image data, copying of the image is prohibited (for example, refer to Japanese Patent laid open Publication No. 2004-201069). The precision of the detection of the ground pattern depends on the resolution of the copying machine, a threshold value used for removing a ground image from an image and the like. Therefore, the ground pattern for preventing copying can not necessarily be detected accurately, and the precision of the detection depends on a combination of the performance of a copying machine and the characteristics of a ground pattern characteristic. Thus, the effect of the ground pattern for preventing copying may not be sufficient. Moreover, in order to prevent illegal copying or the like, the ground pattern should be emphasized visually when an original including the ground pattern is copied. Therefore, it is necessary to control the emphasis processing properly.
In another technique for improving the effect of the copy preventive ground pattern, a watermark is printed together with the copy preventive ground pattern. When detecting the water mark on copying, a measure for copy prevention is activated. In this technique, the measure can be activated without depending on the characteristic of the copying machine. However, a sufficient effect is not assured for a document for which the watermark technique is not introduced.
A ground of an image influences read of the image. Therefore, in order to eliminate the influence of the ground, a print detector disclosed in Japanese Patent laid open Publication No. 2002-15304 surely reads the print printed on the ground by controlling the threshold value for detecting the print according to the density of the ground. This is a technique for changing an image processing parameter for reading characters according to the density of the ground.
However, even if an original is printed by including a copy preventive ground pattern in the image, it may not be possible so far to accurately detect the ground pattern in the original, because the detection performance depends on a combination of the detection performance of an image forming apparatus such as a copying machine or a printer and a feature of the ground pattern. Particularly, it is to be noted that originals to be read are produced by various image forming apparatuses having various printing characteristics. Therefore, it is desirable to ensure that copy preventive ground patterns output from various image forming apparatuses can serve to prevent illegal copy or the like to the maximum. Then a large amount of documents created previously can be used positively and effectively.